The company says several times in the last several years, a 737 engine has momentarily surged without warning. It's well within the limits of the aircraft so there's no immediate danger, but once or twice, it has caused the pilots to return to the airport.
Boeing says, with two exceptions, the unexpected rev of the engines happens to just one of the 737's engines. The engine surges generally last for a few seconds and are extremely rare: in the 46 million times one of these engines has eased up after takeoff, an engine has instead surged just 32 times.
While not ruling out other causes, Boeing says it suspects fuel contamination, possibly linked to aviation fuel from Sea-Tac Airport, because they've traced it to 737 flights in the West, and nowhere else. Alaska Airlines says it has experienced half of those 32 incidents, including once where the pilots returned to the airport.
But Boeing told KOMO News it think it's already countered the problem with a software program that better controls the fuel system during deceleration.
"No incidents have occurred since the software update was implemented," the company said. "However we will continue our root-cause analysis of the fuel supply chain to identify any additional measures to prevent these rare occurrences from happening at all."
Boeing has reminded 737 pilots to pull back smoothly on the throttle once they've taken off and are heading up to cruise altitude -- they say that will also help avoid the problem.